<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .outer {
        height: 100px;
        /* float: left; */
        }
        .left {
        float: left;
        width: 200px;
        height: 100%;
        background: lightcoral;
        }
        .right {
        overflow: auto;
        height: 100%;
        background: lightseagreen;
        }
    </style>
</head>
<body>
    <div class="outer">
        <div class="left">左侧</div>
        <div class="right">右侧</div>
      </div> 
</body>
</html>